diff --git a/hit.c b/hit.c
index 9616bf5..92aa6b7 100644
--- a/hit.c
+++ b/hit.c
@@ -106,7 +106,7 @@ void mm_split_reg(mm_reg1_t *r, mm_reg1_t *r2, int n, int qlen, mm128_t *a)
 	r->split |= 1, r2->split |= 2;
 }
 
-void mm_set_parent(void *km, float mask_level, int n, mm_reg1_t *r, int sub_diff) // and compute mm_reg1_t::subsc
+void mm_set_parent(void *km, float mask_level, int n, mm_reg1_t *r, int sub_diff, int hard_mask_level) // and compute mm_reg1_t::subsc
 {
 	int i, j, k, *w;
 	uint64_t *cov;
@@ -118,6 +118,7 @@ void mm_set_parent(void *km, float mask_level, int n, mm_reg1_t *r, int sub_diff
 	for (i = 1, k = 1; i < n; ++i) {
 		mm_reg1_t *ri = &r[i];
 		int si = ri->qs, ei = ri->qe, n_cov = 0, uncov_len = 0;
+		if (hard_mask_level) goto skip_uncov;
 		for (j = 0; j < k; ++j) { // traverse existing primary hits to find overlapping hits
 			mm_reg1_t *rp = &r[w[j]];
 			int sj = rp->qs, ej = rp->qe;
@@ -137,6 +138,7 @@ void mm_set_parent(void *km, float mask_level, int n, mm_reg1_t *r, int sub_diff
 			}
 			if (ei > x) uncov_len += ei - x;
 		}
+skip_uncov:
 		for (j = 0; j < k; ++j) { // traverse existing primary hits again
 			mm_reg1_t *rp = &r[w[j]];
 			int sj = rp->qs, ej = rp->qe, min, max, ol;
diff --git a/main.c b/main.c
index b31d718..97bc90f 100644
--- a/main.c
+++ b/main.c
@@ -6,7 +6,7 @@
 #include "mmpriv.h"
 #include "ketopt.h"
 
-#define MM_VERSION "2.12-r845-dirty"
+#define MM_VERSION "2.12-r846-dirty"
 
 #ifdef __linux__
 #include <sys/resource.h>
@@ -59,6 +59,7 @@ static ko_longopt_t long_options[] = {
 	{ "paf-no-hit",     ko_no_argument,       333 },
 	{ "split-prefix",   ko_required_argument, 334 },
 	{ "no-end-flt",     ko_no_argument,       335 },
+	{ "hard-mask-level",ko_no_argument,       336 },
 	{ "help",           ko_no_argument,       'h' },
 	{ "max-intron-len", ko_required_argument, 'G' },
 	{ "version",        ko_no_argument,       'V' },
@@ -188,6 +189,7 @@ int main(int argc, char *argv[])
 		else if (c == 333) opt.flag |= MM_F_PAF_NO_HIT; // --paf-no-hit
 		else if (c == 334) opt.split_prefix = o.arg; // --split-prefix
 		else if (c == 335) opt.flag |= MM_F_NO_END_FLT; // --no-end-flt
+		else if (c == 336) opt.flag |= MM_F_HARD_MLEVEL; // --hard-mask-level
 		else if (c == 314) { // --frag
 			yes_or_no(&opt, MM_F_FRAG_MODE, o.longidx, o.arg, 1);
 		} else if (c == 315) { // --secondary
diff --git a/map.c b/map.c
index 9c99824..6c21103 100644
--- a/map.c
+++ b/map.c
@@ -248,7 +248,7 @@ static mm128_t *collect_seed_hits(void *km, const mm_mapopt_t *opt, int max_occ,
 static void chain_post(const mm_mapopt_t *opt, int max_chain_gap_ref, const mm_idx_t *mi, void *km, int qlen, int n_segs, const int *qlens, int *n_regs, mm_reg1_t *regs, mm128_t *a)
 {
 	if (!(opt->flag & MM_F_ALL_CHAINS)) { // don't choose primary mapping(s)
-		mm_set_parent(km, opt->mask_level, *n_regs, regs, opt->a * 2 + opt->b);
+		mm_set_parent(km, opt->mask_level, *n_regs, regs, opt->a * 2 + opt->b, opt->flag&MM_F_HARD_MLEVEL);
 		if (n_segs <= 1) mm_select_sub(km, opt->pri_ratio, mi->k*2, opt->best_n, n_regs, regs);
 		else mm_select_sub_multi(km, opt->pri_ratio, 0.2f, 0.7f, max_chain_gap_ref, mi->k*2, opt->best_n, n_segs, qlens, n_regs, regs);
 		if (!(opt->flag & (MM_F_SPLICE|MM_F_SR|MM_F_NO_LJOIN))) // long join not working well without primary chains
@@ -261,7 +261,7 @@ static mm_reg1_t *align_regs(const mm_mapopt_t *opt, const mm_idx_t *mi, void *k
 	if (!(opt->flag & MM_F_CIGAR)) return regs;
 	regs = mm_align_skeleton(km, opt, mi, qlen, seq, n_regs, regs, a); // this calls mm_filter_regs()
 	if (!(opt->flag & MM_F_ALL_CHAINS)) { // don't choose primary mapping(s)
-		mm_set_parent(km, opt->mask_level, *n_regs, regs, opt->a * 2 + opt->b);
+		mm_set_parent(km, opt->mask_level, *n_regs, regs, opt->a * 2 + opt->b, opt->flag&MM_F_HARD_MLEVEL);
 		mm_select_sub(km, opt->pri_ratio, mi->k*2, opt->best_n, n_regs, regs);
 		mm_set_sam_pri(*n_regs, regs);
 	}
@@ -359,7 +359,7 @@ void mm_map_frag(const mm_idx_t *mi, int n_segs, const int *qlens, const char **
 		seg = mm_seg_gen(b->km, hash, n_segs, qlens, n_regs0, regs0, n_regs, regs, a); // split fragment chain to separate segment chains
 		free(regs0);
 		for (i = 0; i < n_segs; ++i) {
-			mm_set_parent(b->km, opt->mask_level, n_regs[i], regs[i], opt->a * 2 + opt->b); // update mm_reg1_t::parent
+			mm_set_parent(b->km, opt->mask_level, n_regs[i], regs[i], opt->a * 2 + opt->b, opt->flag&MM_F_HARD_MLEVEL); // update mm_reg1_t::parent
 			regs[i] = align_regs(opt, mi, b->km, qlens[i], seqs[i], &n_regs[i], regs[i], seg[i].a);
 			mm_set_mapq(b->km, n_regs[i], regs[i], opt->min_chain_score, opt->a, rep_len, is_sr);
 		}
@@ -502,7 +502,7 @@ static void merge_hits(step_t *s)
 				}
 			}
 			mm_hit_sort(km, &s->n_reg[k], s->reg[k]);
-			mm_set_parent(km, opt->mask_level, s->n_reg[k], s->reg[k], opt->a * 2 + opt->b);
+			mm_set_parent(km, opt->mask_level, s->n_reg[k], s->reg[k], opt->a * 2 + opt->b, opt->flag&MM_F_HARD_MLEVEL);
 			if (!(opt->flag & MM_F_ALL_CHAINS)) {
 				mm_select_sub(km, opt->pri_ratio, s->p->mi->k*2, opt->best_n, &s->n_reg[k], s->reg[k]);
 				mm_set_sam_pri(s->n_reg[k], s->reg[k]);
diff --git a/minimap.h b/minimap.h
index 27ba911..20328c8 100644
--- a/minimap.h
+++ b/minimap.h
@@ -34,6 +34,7 @@
 #define MM_F_EQX           0x4000000 // use =/X instead of M
 #define MM_F_PAF_NO_HIT    0x8000000 // output unmapped reads to PAF
 #define MM_F_NO_END_FLT    0x10000000
+#define MM_F_HARD_MLEVEL   0x20000000
 
 #define MM_I_HPC          0x1
 #define MM_I_NO_SEQ       0x2
diff --git a/minimap2.1 b/minimap2.1
index cea2e6e..6f68ddc 100644
--- a/minimap2.1
+++ b/minimap2.1
@@ -193,7 +193,7 @@ Primarily used for all-vs-all read overlapping.
 .BI -p \ FLOAT
 Minimal secondary-to-primary score ratio to output secondary mappings [0.8].
 Between two chains overlaping over half of the shorter chain (controlled by
-.BR --mask-level ),
+.BR -M ),
 the chain with a lower score is secondary to the chain with a higher score.
 If the ratio of the scores is below
 .IR FLOAT ,
@@ -226,6 +226,11 @@ Mark as secondary a chain that overlaps with a better chain by
 .I FLOAT
 or more of the shorter chain [0.5]
 .TP
+.B --hard-mask-level
+Honor option
+.B -M
+and disable a heurstic to save unmapped subsequences.
+.TP
 .BI --max-chain-skip \ INT
 A heuristics that stops chaining early [50]. Minimap2 uses dynamic programming
 for chaining. The time complexity is quadratic in the number of seeds. This
diff --git a/mmpriv.h b/mmpriv.h
index f847445..2688bca 100644
--- a/mmpriv.h
+++ b/mmpriv.h
@@ -75,7 +75,7 @@ void mm_split_reg(mm_reg1_t *r, mm_reg1_t *r2, int n, int qlen, mm128_t *a);
 void mm_sync_regs(void *km, int n_regs, mm_reg1_t *regs);
 int mm_squeeze_a(void *km, int n_regs, mm_reg1_t *regs, mm128_t *a);
 int mm_set_sam_pri(int n, mm_reg1_t *r);
-void mm_set_parent(void *km, float mask_level, int n, mm_reg1_t *r, int sub_diff);
+void mm_set_parent(void *km, float mask_level, int n, mm_reg1_t *r, int sub_diff, int hard_mask_level);
 void mm_select_sub(void *km, float pri_ratio, int min_diff, int best_n, int *n_, mm_reg1_t *r);
 void mm_select_sub_multi(void *km, float pri_ratio, float pri1, float pri2, int max_gap_ref, int min_diff, int best_n, int n_segs, const int *qlens, int *n_, mm_reg1_t *r);
 void mm_filter_regs(const mm_mapopt_t *opt, int qlen, int *n_regs, mm_reg1_t *regs);